The performance of lubricating oils and greases (hereinafter inclusively referred to as “lubricant”) is largely determined by the properties of the base oil used as the major ingredient when the lubricant is used under high vacuum or ultra high vacuum and high temperatures. For example, the conventional base oils widely used for the lubricant, such as mineral oils, ester type oils, poly α-olefins, alkylphenyl ether type oils and the like, show high vapor pressure. In light of this, it is difficult to use lubricants containing the base oils mentioned above under a high vacuum. Instead, perfluoroalkyl ether (PFAE), tris(2-octyldodecyl)cyclopentane or the like may be used as a base oil with low vapor pressure.
Currently, particular attention has been paid to the ionic liquid as a lubricant base oil having higher resistance to vacuum and higher heat resistance than the above-mentioned base oils (WO 2005/035702, JP 2007-297287 A and JP 2005-154755 A). There are many different types of ionic liquids, most of which exhibit water solubility as is the nature of ionic liquids. However, the water-soluble lubricants are disadvantageous because such lubricants easily dissolve into water to induce leakage and have an adverse effect on rust prevention properties. In consideration of the above, an ionic liquid used as a lubricant base oil is basically required to be insoluble in water. Further, ionic liquids have a negative effect on the rust prevention properties. To overcome the above-mentioned drawback, a variety of rust inhibitors are used as disclosed in JP 2006-291011 A, JP 2009-29981 A, JP 2009-249585 A and JP 2009-242765 A. However, sufficient rust prevention effects cannot be obtained through any of the above references, and all of the aforementioned conventional lubricants are still unsatisfactory.